Objective To evaluate if 18F-FLT PET imaging could be used as a new clinical method to predict tumor radiosensitivity.Methods MDA-MB-231 and LN229 cells were irradiated with doses of 0,8 and 16 Gy of 6 MV photon energy,then soft agar assay and cellular uptake of 18F-FLT were performed on the 2 cell lines.The t test and one-way analysis of variance were used for the two groups and data before and after irradiation.The MDA-MB-231 and LN229 tumor xenografts were prepared by injecting the tumor cells into the right limbs of female BALB/c nu/nu mice.Once tumors reached a diameter of 10 mm,the two types of mice were divided randomly into 3 groups (20 mice per group) according to the irradiation doses (0,8 and 16 Gy).After irradiation,18F-FLT PET imaging and immunohistochemical staining were conducted.Then correlations between 18F-FLT SUVtumor/SUVmuscle ratio (T/M ratio) and TK1 labeling index percentages (LITK1) were tested using linear correlation analysis.Results The survival fraction of MDA-MB-231 and LN229 cells after irradiated with 8 Gy were (59.73 ± 4.3) ％ and (93.41 ± 3.75) ％,respectively (t =-13.20,P ＜ 0.001).When the dose increased to 16 Gy,the survival fraction decreased to (43.57 ±4.06) ％ and (81.77 ± 4.42) ％,respectively(t =-14.24,P ＜ 0.001).In MDA-MB-231 cells,the cellular uptake of 18F-FLT after irradiation with 8 Gy declined rapidly to (18.32 ± 1.38) kBq/105 cells ((128.22 ± 8.24) kBq/105 cells with the dose of 0 Gy,F =266.41,P ＜ 0.01),and maintained this low level till 72 h.For the LN229 cells,the cellular uptake decreased to (9.87 ± 1.30) kBq/105 cells after 8 Gy irradiation ((134.88 ± 6.59) kBq/105 cells with the dose of 0 Gy,F =346.06,P ＜ 0.01),then increased gradually to (127.17 ± 9.08) kBq/105 cells at 72 h (F =346.06,P ＞ 0.05).The dynamic changes of 18F-FLT cellular uptake in the two cells had the same pattern after being treated with 16 Gy irradiation.In the 18F-FLT PET image of MDA-MB-231 tumor mice after 8 Gy radiotherapy,the T/M ratio decreased to 0.78 ± 0.39 at the first day,but it was 2.84 ± 0.29 before radiotherapy (F =39.78,P ＜0.01).Then the ratio increased slowly,and it was still lower than the baseline at 7 d after radiation (F =39.78,P ＜0.01).The same pattern could be seen in the group of 16 Gy irradiation.In LN229 tumor mice treatment with 8 Gy irradiation,the T/M ratio increased to 2.41 ±0.47 at the first day,and it was 1.58 ±0.29 before radiotherapy (F =34.01,P ＜ 0.05).The ratio decreased steadily to 0.66 ± 0.32 (F =34.01,P＜0.05) at 7 d after radiotherapy.However,in the treatment group with 16 Gy,the T/M ratio decreased gradually and reached 0.44 ± 0.22 at 7 d (F =41.85,P ＜ 0.01).A correlation was found between 18F-FLT T/M ratio and LITK1 (8 Gy:r=0.67,0.73; 16 Gy:r=0.73,0.69; all P＜0.01) in both tumor models.Conclusion 18F-FLT PET imaging may be used as a new assay to predict tumor radiosensitivity,but further investigation is needed before clinical application.